1 . Field of the Invention
The present invention relates to the transmission of information over a communications path. More particularly, the present invention relates to the communications of high bandwidths information over networks of varying types.
2 . Art Background
Until recently, telecommunications and computing were considered to be entirely separate disciplines. Telecommunications was analog and done in real time whereas computing was digital and performed at a rate determined by the processing speed of a computer. Today, such technologies as speech processing, electronic mail and facsimile have blurred these lines. In the coming years, computing and telecommunications will become almost indistinguishable in a race to support a broad range of new multimedia (i.e., voice, video and data) applications. These applications are made possible by emerging digital-processing technologies, which include: compressed audio (both high fidelity audio and speech), high resolution still images, and compressed video. The emerging technologies will allow for collaboration at a distance, including video conferencing.
Of these technologies, video is particularly exciting in terms of its potential applications. But video is also the most demanding in terms of processing power and sheer volume of data to be processed. Uncompressed digital video requires somewhere between 50 and 200 Mb/s (megabits per second) to support the real-time transmission of standard television quality images. This makes impractical the widespread use of uncompressed digital video in telecommunications applications.
Fortunately, there is considerable redundancy in video data, both in terms of information theory and human perception. This redundancy allows for the compression of digital video sequences into lower transmission rates. For some time, researchers have been aware of a variety of techniques that can be used to compress video data sequences anywhere from 2:1 to 1000:1, depending on the quality required by the application. Until recently, however, it was not practical to incorporate these techniques into low cost video-based applications.
A number of standards have been recently developed for such activities as video conferencing, the transmission and storage of standard high quality still images, as well as standards for interactive video playback to provide interoperability between numerous communications points. The standards recognize a need for quality video compression to reduce the tremendous amount of data required for the transmission of video information.
Two important methods of data compression for video information are used widely throughout the various standards for video communication. These are the concepts of frame differencing and motion compensation. Frame differencing recognizes that a normal video sequence has little variation from one frame to the next. If, instead of coding each frame, only the differences between a frame and the previous frame are coded, then the amount of information needed to describe the new frame will be dramatically reduced. Motion compensation recognizes that much of the difference that does occur between successive frames can be characterized as a simple translation of motion, caused either by the moving of objects in the scene or by a pan of the field of view. Rather than form a simple difference between blocks in a current frame and the same block in the previous frame, the area around those blocks can be searched in the previous frame to find an offset block than more closely matches the block of the current frame. Once a best match has been identified, the difference between a reference block in the current frame and the best match in the previous frame are coded to produce a vector that describes the offset of the best match. This motion vector then can be used with the previous frame to produce the equivalent of what the current frame should be. These methods, and others are incorporated into systems which make possible the rapid transmission of real-time video information.
As the worlds of telecommunications and computers blend closely together, the telecommunications aspects of communications will have to contend with some of the constraints of the computer world. Particularly, video conferencing over existing computer networks will prove a challenge in that maintaining real time information communication over traffic-burdened existing network protocols may prove insurmountable.
Current video algorithms assume a nearly constant bandwidth availability for the encoding of video information. This is evidenced by the use of only a single output buffer for traditional video encoder output. It is common to use the output buffer fullness as a feedback parameter for encoding subsequent images; i.e., with higher or lower levels of quantization. A well-known effect resulting from using a single output buffer is called "bit-bang" where the output buffer is over depleted by the interface to the communications channel, causing the feedback loop to indicate that the buffer can handle lots of data, which in turn causes the video compression algorithm to under optimize the subsequent image coding. The user perceives the bit-bang as an uneven quality and frame rate.
To alleviate bit-bang, the typical approach has been to limit the amount of data pulled out from the encoder video output buffer to a fraction of the total size of the output buffer; 10% to 30% is typical. This approach keeps the feedback indicator rather small, and encoding more uniform. The underlying assumption of this approach is that the communications channel will usually not be changing rapidly. Exceptions are caused by connectivity interruptions, such as burst errors, which are handled strictly as exceptions to the call. In a local area network (LAN), or other collision-sensing multiple access channel, or in other networks with burst characteristics (such as noisy RF channels), this underlying assumption no longer holds. Over these sorts of communications channels, unanticipated transmission delays may result in bit-bang problems which are not so readily overcome by limiting the size of the feedback buffer. Thus, video jerkiness will result in real-time video communication over such channels. It would be advantageous, and is therefore an object of the present invention to provide a video transmission mechanism which can be accommodated on such potential bursty networks.